This invention relates to markers for use in electronic article surveillance systems and, in particular, to magnetic markers of the type which comprise magnetostrictive materials.
U.S. Pat. Nos. 4,510,489 and 4,510,490, both issued Apr. 9, 1985, describe a magnetomechanical tag formed from a magnetostrictive ferromagnetic element or strip. Disposed adjacent to the magnetostrictive element is a further element comprised of a hard magnetic material, such as Vicalloy or Arnokrome, which acts as a biasing mechanism. Arming or activating the magnetostrictive element is accomplished by magnetization of the ferromagnetic biasing element, thereby causing a DC magnetic field to be placed along the magnetostrictive element.
As described in the '489 patent, when the magnetostrictive element is biased by the DC magnetic field, subsequent application of an AC magnetic field to the magnetostrictive element causes the element to store energy mechanically and magnetically, via magnetostriction or magnetomechanical coupling. This energy is alternately stored and released at the frequency of the applied AC field.
Furthermore, the storage and release of such magnetomechanical energy is maximized at the material's magnetomechanical resonance and minimal at its anti-resonance and results in a detectable magnetic signal being generated by the magnetostrictive element. The strength of the generated magnetic signal depends upon the magnetomechanical coupling factor of the element. The latter factor, in turn, depends upon a number of criteria including the level of the DC bias field, the level of internal stress present in the element and the level and direction of any magnetic anisotropy in the element.
The '489 patent teaches that the coupling factor can be enhanced by creating or inducing a magnetic anisotropy in the magnetostrictive element in a direction transverse to its length. The direction of the induced anisotropy defines a so-called "easy axis" for the element. The '489 patent discloses that one way of establishing the induced anisotropy is by annealing the element in the presence of a magnetic field. The latter field magnetically saturates the element and is applied in a direction perpendicular to the length of the element. The '489 patent also teaches that the induced anisotropy can be established during formation of the element by casting.
As above-noted, the magnetostrictive element of the '489 marker is biased with a DC bias field via the field from the adjacent ferromagnetic element. Typically, the strength of the DC bias field is selected so as to cause the magnetostrictive element to operate near the so-called "knee" of its M-H curve. The '489 patent also proposes that the D.C. bias field can be provided by remanent flux in the magnetostrictive element but there is no disclosure as to how this can be accomplished.
The marker of the 489 patent has several advantages over other magnetic markers in terms of uniqueness and large signal to noise ratio. However, since the marker of '489 patent uses a magnetic biasing element, this complicates the mechanical design and adds cost to the marker structure. In addition, remote deactivation of the marker using a magnetic field requires a large threshold field (typically 150 Oe) and, therefore, high power. As a result, remote deactivation has generally not been used and deactivation has been limited to contact deactivation by a multipole magnet or bulk deactivation in a small volume of about 1 cu. ft.
Also, in the marker of the '489 patent, an attractive force exists between the magnetostrictive element and the ferromagnetic biasing element. This force results in a mechanical damping of the magnetostrictive element which is undesirable. Additionally, the geometry of the biasing element results in the generation of a non-uniform biasing field.
It is therefore a primary object of the present invention to provide a magnetomechanical marker which avoids the need to use a separate biasing element.
It is a further object of the invention to provide a magnetomechanical marker which can be more easily remotely deactivated.